The invention relates to a video signal multiplying circuit having a first terminal for receiving a video signal, a second terminal for receiving a multiplying signal and third terminal for supplying a video signal multiplied by a signal multiplier, which terminals are coupled to respective corresponding terminals of the signal multiplier, the signals occurring with alternate scanning periods and blanking periods.
A circuit of this type is disclosed in U.S. Pat. No. 3,743,772. The circuit described in said patent is used with a television camera for effecting a shading correction. Such a correction is required when a uniformly illuminated recording area of a pick-up arrangement in the television camera results in a video or picture signal which, on display, does not result in a uniformly luminescing television picture. The multiplying signal is derived for correcting the shading errors, so that the multiplied video signal results on display in a television picture which does indeed luminesce uniformly.
In addition to the shading error correction derived for uniform illumination, a further shading error correction must be effected, more specifically at the level which is commonly referred to as the black level of the video signal. As a rule this black level is contained in at least a portion of the signal blanking periods. The black level should be present in the video signal in the scanning periods with a zero value in those positions where the recording area is not illuminated. In practice it has been found that in the absence of illumination, the pick-up arrangement still supplies a picture signal with a value deviating from black level. To correct the resultant shading error on display, a correction signal is subtracted from the video signal. Thus, two shading error corrections can be distinguished, namely the subtractive correction at the black level and the multiplicative correction at the video signal values which deviate from the black level.
For signal multiplying circuits, it generally holds that the signal multipliers are beset with an unbalance which changes in time under the influence of, for example, aging phenomena and temperature fluctuations. Such an unbalance results in the signal multiplication not occurring in an ideal manner. Instead of a multiplied video signal equal to zero occurring at the third terminal when the video signal at the first terminal or the multiplying signal at the second terminal are equal to zero, there usually occurs at the third terminal a residue of the non-zero signal. In addition, the signal multiplication at signal values deviating from zero is not effected in a pure manner, but the multiplied signal at the third terminal is formed from a purely multiplicative component and an additive or subtractive offset component superimposed thereon. Both deviations of the purely multiplicative signal processing operation vary with time, so that a more or less regularly repeated set-up of the signal multiplying circuit is required.
In the case of the described shading error correction with the subtractive correction at the black level as a zero value and the multiplicative correction at the signal values deviating therefrom, it has been found that after an optimum set-up of the subtractive correction and thereafter an optimum set-up of the multiplicative correction, the first set-up is no longer to optimal and must be repeated, whereafter the second set-up deviates from the optimum and must likewise be repeated, etc. It has been found in practice that due to the unbalance at the signal multiplier, the two set-up operations during the shading error correction must alternately be repeated several times. Added to this is the effect that the unbalance deteriorates with time, so that for an operator the set-up of the shading error correction is a time-consuming matter which requires a great deal of attention.